1. Field of the Invention
The present invention relates generally to a method and unit for driving a piezoelectric transformer used in various high-voltage transformer assemblies.
2. Related Background Art
FIG. 14 shows a configuration of a Rosen-type piezoelectric transformer that is a typical configuration of conventional piezoelectric transformers. This piezoelectric transformer has advantages of, for example, having a smaller size than that of an electromagnetic transformer, being incombustible, and generating no noise caused by electromagnetic induction.
In FIG. 14, a portion indicated with numeral 1 is a low impedance portion of the piezoelectric transformer and functions as an input part when the piezoelectric transformer is used for voltage step-up. The low impedance portion 1 is polarized in the thickness direction (PD), and electrodes 3U and 3D are disposed on its principal planes in the thickness direction. On the other hand, a portion indicated with a numeral 2 is a high impedance portion and functions as an output part when the piezoelectric transformer is used for voltage step-up. The high impedance portion 2 is polarized in the longitudinal direction (PL) and an electrode 4 is disposed on an end face in the longitudinal direction.
A piezoelectric transformer as shown in FIG. 14 has characteristics that a very high step-up ratio can be obtained under an infinite load and the step-up ratio decreases with reduction in load. Due to those characteristics, recently such a piezoelectric transformer has been used as a power supply for a cold-cathode tube. An inverter with a piezoelectric transformer can generate a high voltage efficiently.
FIG. 15 is a block diagram showing a configuration of a conventional self-oscillation type drive for a piezoelectric transformer. In FIG. 15, numeral 13 indicates a variable oscillation circuit for producing a variable-frequency voltage signal. A voltage signal output from the variable oscillation circuit 13 generally has a pulse waveform. A high-frequency component in the voltage signal is removed by a wave shaping circuit 11 and thus the voltage signal is converted into an AC signal with a substantially sinusoidal waveform. An output signal from the wave shaping circuit 11 is converted to a voltage, the voltage is amplified to a sufficient level to drive a piezoelectric transformer 10 by a drive circuit 12, and then the voltage thus amplified is input to one primary side electrode 3U of the piezoelectric transformer 10. The other primary side electrode 3D of the piezoelectric transformer 10 is connected to a ground potential. A voltage stepped up by a piezoelectric effect of the piezoelectric transformer 10 is output from the secondary side electrode 4.
A high voltage output from the secondary side electrode is applied to a series circuit including a cold-cathode tube 17 and a feedback resistance 18 and to an overvoltage protection circuit section 20. The overvoltage protection circuit section 20 includes resistances 19a and 19b and a comparing circuit 15. The comparing circuit 15 compares a voltage obtained through division by the resistances 19a and 19b with a reference voltage Vref1. The comparing circuit 15 outputs a signal to an oscillation control circuit 14 so that the high voltage output from the secondary side electrode 4 of the piezoelectric transformer is prevented from rising beyond a preset voltage determined depending on the reference voltage Vref1. This overvoltage protection circuit section 20 does not operate during emission by the cold-cathode tube 17.
A voltage generated at both ends of the feedback resistance 18 by a current flowing in the series circuit including the cold-cathode tube 17 and the feedback resistance 18 is applied to one input terminal of a comparing circuit 16 as a feedback voltage. The comparing circuit 16 compares the feedback voltage with a reference voltage Vref2 applied to the other input terminal and sends a signal to the oscillation control circuit 14 so that a substantially constant current flows in the cold-cathode tube 17.
The oscillation control circuit 14 outputs a signal to the variable oscillation circuit 13 to allow the variable oscillation circuit 13 to oscillate at a frequency corresponding to the output signal from the comparing circuit 16. This comparing circuit 16 does not operate before a start of emission by the cold-cathode tube 17.
Thus, the cold-cathode tube 17 emits light stably. In the case where the piezoelectric transformer is driven by a self-oscillation system, even when the resonance frequency of the piezoelectric transformer varies depending on temperatures, a drive frequency automatically follows the resonance frequency.
As described above, an inverter with a configuration using a piezoelectric transformer allows driving of the piezoelectric transformer to be controlled so that a constant current flows in the cold-cathode tube 17.
In order to prevent variations in luminance of the cold-cathode tube, for example, the following drive methods have been proposed. In one driving method, as shown in FIG. 9, two piezoelectric transformers 22 and 23 are driven in parallel with each other and a cold-cathode tube 21 is allowed to emit light with two AC voltage signals V1 and V2 whose phases are different from each other by 180xc2x0. In another driving method, using a piezoelectric transformer 61 with a configuration shown in FIG. 10, two output electrodes 4L and 4R of the piezoelectric transformer 61 are connected to two input terminals 641 and 642 of a cold-cathode tube 64, respectively, as shown in FIG. 11.
In such drives, in an operation carried out in the drive shown in FIG. 15, it is necessary to feedback a current flowing in the cold-cathode tube to control the frequency and voltage. Alternatively, feedback is carried out through detection of luminance of the cold-cathode tube.
In order to obtain a constant luminance of the cold-cathode tube, a current flowing in the cold-cathode tube is controlled through detection of an output current or voltage of the piezoelectric transformer (for example, by the output current detecting circuit 24 or the output voltage comparing circuit 25 shown in FIG. 9) or through detection of a current flowing in a reflector.
In the conventional drives for a piezoelectric transformer described above, the current flowing in the cold-cathode tube is controlled by the feedback of a voltage detected by the feedback resistance 18 (FIG. 15) connected to the cold-cathode tube.
However, due to stray capacitance Cx (FIGS. 9 and 11) between a cold-cathode tube and a reflector (a reflector 26 shown in FIG. 9, a reflector 65 shown in FIG. 11), a current flows out to the reflector from the cold-cathode tube. As a result, there has been a problem of variations in, luminance of the cold-cathode tube.
In order to solve this problem, JP 11(1999)-8087 A proposes a means for inputting voltages whose phases are different by 180xc2x0 from respective ends of a cold-cathode tube. As shown in FIG. 12A, however, when voltages are applied to one cold-cathode tube 51 as shown in FIG. 12A or to two cold-cathode tubes 51 and 52 connected with each other in series as shown in FIG. 13A, a current flows out from the cold-cathode tube to the reflector (a current Ixa on the (+) side shown in FIG. 12B and a current Ixb on the (+) side shown in FIG. 12C) on a higher-voltage side (the side to which a voltage V1 is applied during a period ta, the side to which a voltage V2 is applied during a period tb). On the other hand, a current flows into the cold-cathode tube from the reflector (a current Ixa on the (xe2x88x92) side shown in FIG. 12B and a current Ixb on the (xe2x88x92) side shown in FIG. 12C) on a lower-voltage side (the side to which a voltage V1 is applied during a period tb, the side to which a voltage V2 is applied during a period ta).
Therefore, an output current from the piezoelectric transformer contains both a current Ia flowing only in the cold-cathode tube and leakage currents Ixa and Ixb (Ix) flowing in the stray capacitance Cx. During emission by the cold-cathode tube, the cold-cathode tube is handled as a resistive load. Therefore, a current participating in the luminance of the cold-cathode tube is only an active current Ia=Icosxcex8(xcex8 indicates a phase difference between an output voltage and an output current from the piezoelectric transformer as shown in FIG. 16) of a current (I) output from the piezoelectric transformer. In other words, the leakage current Ix flowing in the reflector through the stray capacitance Cx becomes a reactive current and thus does not participate in the luminance of the cold-cathode tube.
Hence, in the drives for a piezoelectric transformer with the configurations as shown in FIGS. 9 and 11, the output current detecting circuit 24, 62 detects the current Ia flowing in the cold-cathode tube together with the leakage current Ix caused by the stray capacitance Cx formed by, for example, the cold-cathode tube and the reflector. If the stray capacitance Cx formed by the reflector were constant, the current flowing in the cold-cathode tube would be controlled to be constant with consideration given to the constant capacitance Cx. However, the stray capacitance Cx varies and it therefore is difficult to control the current Ia flowing in the cold-cathode tube so that the current Ia is constant. This causes variations in luminance among inverters, or the like. In addition, similarly in the case of a drive with two piezoelectric transformers, it is difficult to control a tube current.
In JP 11(1999)-27955 A, a leakage current and a tube current are detected by a stray capacitance current detecting circuit and a tube current detecting circuit, respectively, and thus a tube current is controlled. In the method disclosed in JP 11(1999)-27955 A, however, in a piezoelectric transformer allowing an output voltage to be constant by control of a drive frequency, the impedance depending on stray capacitance varies when the frequency of a leakage current caused by the stray capacitance varies. Accordingly, the magnitude of the leakage current varies. As a result, when a circuit is intended to be configured with consideration also given to an influence of the frequency, a complicated control circuit is required.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method and unit for driving a small high-efficiency piezoelectric transformer allowing a cold-cathode tube to have a stable luminance through removal of an influence of a reactive current as a leakage current caused by stray capacitance between the cold-cathode tube and a reflector and through accurate control to obtain a constant tube current.
In order to achieve the above-mentioned object, a first method of driving a piezoelectric transformer according to the present invention includes: stepping up a voltage input from a primary terminal of a piezoelectric transformer by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect to two terminals of a cold-cathode tube from two secondary terminals of the piezoelectric transformer; detecting a phase difference between the voltage applied to the cold-cathode tube and a current flowing in the cold-cathode tube; detecting an active current flowing in the cold-cathode tube based on the phase difference; comparing the active current with a predetermined set value; and controlling the driving of the piezoelectric transformer so that the active current flowing in the cold-cathode tube has a value equal to the predetermined set value.
In order to achieve the above-mentioned object, a second method of driving piezoelectric transformers according to the present invention includes: inputting a voltage from a primary terminal of a first piezoelectric transformer by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect to one terminal of a cold-cathode tube from a secondary terminal of the first piezoelectric transformer; inputting a voltage from a primary terminal of a second piezoelectric transformer by using the piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect to the other terminal of the cold-cathode tube from a secondary terminal of the second piezoelectric transformer; detecting a phase difference between the voltage applied to the cold-cathode tube and a current flowing in the cold-cathode tube; detecting an active current flowing in the cold-cathode tube based on the phase difference; comparing the active current with a predetermined set value; and controlling driving of the first and second piezoelectric transformers so that the active current flowing in the cold-cathode tube has a value equal to the predetermined set value.
In the first and second driving methods, preferably, the cold-cathode tube includes one or more cold-cathode tubes connected in series.
In order to achieve the above-mentioned object, a first drive for a piezoelectric transformer according to the present invention includes: a piezoelectric transformer for inputting a voltage from its primary terminal by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect from its two secondary terminals; a drive circuit for driving the piezoelectric transformer; a variable oscillation circuit for outputting a variable-frequency voltage to the drive circuit; a cold-cathode tube with two terminals to which the voltage output from the two secondary terminals of the piezoelectric transformer is applied; a current detecting circuit for detecting a current flowing in the cold-cathode tube; a voltage detecting circuit for detecting the voltage applied to the cold-cathode tube; a phase difference detecting circuit for detecting a phase difference between a current signal output from the current detecting circuit and a voltage signal output from the voltage detecting circuit; an active current detecting circuit for detecting an active current flowing in the cold-cathode tube based on the current signal output from the current detecting circuit and the phase difference detected in the phase difference detecting circuit; and an oscillation control circuit for comparing the active current detected in the active current detecting circuit with a predetermined set value and controlling an oscillation frequency of the variable oscillation circuit so that the active current has a value equal to the predetermined set value.
In order to achieve the above-mentioned object, a second drive for piezoelectric transformers according to the present invention includes: a first piezoelectric transformer for inputting a voltage from its primary terminal by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect from its secondary terminal; a second piezoelectric transformer for inputting a voltage from its primary terminal by using the piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect from its secondary terminal; drive circuits for driving the first and second piezoelectric transformers with signals whose phases are different from each other by 180xc2x0, respectively; variable oscillation circuits for outputting variable-frequency voltages to the drive circuits, respectively; a cold-cathode tube with one terminal to which the voltage output from the secondary terminal of the first piezoelectric transformer is applied and the other terminal to which the voltage output from the secondary terminal of the second piezoelectric transformer is applied; a current detecting circuit for detecting a current flowing in the cold-cathode tube; a voltage detecting circuit for detecting the voltage applied to the cold-cathode tube; a phase difference detecting circuit for detecting a phase difference between a current signal output from the current detecting circuit and a voltage signal output from the voltage detecting circuit; an active current detecting circuit for detecting an active current flowing in the cold-cathode tube based on the current signal output from the current detecting circuit and the phase difference detected in the phase difference detecting circuit; and an oscillation control circuit for comparing the active current detected in the active current detecting circuit with a predetermined set value and controlling oscillation frequencies of the variable oscillation circuits so that the active current has a value equal to the predetermined set value.
In order to achieve the above-mentioned object, a third method of driving piezoelectric transformers according to the present invention includes: inputting a voltage from a primary terminal of a first piezoelectric transformer by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect to one terminal of a cold-cathode tube from a secondary terminal of the first piezoelectric transformer; inputting, by using the piezoelectric effect, a voltage from a primary terminal of a second piezoelectric transformer having a phase identical with that of the voltage input to the first piezoelectric transformer, and outputting a voltage stepped up by using the piezoelectric effect having a different phase from that of the voltage output from the first piezoelectric transformer by 180xc2x0 to the other terminal of the cold-cathode tube from a secondary terminal of the second piezoelectric transformer; and allowing the cold-cathode tube to emit light.
Preferably, the third driving method further includes: detecting a phase difference between the voltage applied to the cold-cathode tube and a current flowing in the cold-cathode tube; detecting an active current flowing in the cold-cathode tube based on the phase difference; comparing the active current with a predetermined set value; and controlling driving of the first and second piezoelectric transformers so that the active current flowing in the cold-cathode tube has a value equal to the predetermined set value.
In order to achieve the above-mentioned object, a third drive for piezoelectric transformers according to the present invention includes: a first piezoelectric transformer for inputting a voltage from its primary terminal by using a piezoelectric effect and outputting a voltage stepped up by using the piezoelectric effect from its secondary terminal; a second piezoelectric transformer for inputting, by using the piezoelectric effect, a voltage from its primary terminal having a phase identical with that of the voltage input to the first piezoelectric transformer, and outputting, from its secondary terminal, a voltage stepped up by using the piezoelectric effect having a different phase from that of the voltage output from the first piezoelectric transformer by 180xc2x0; drive circuits for driving the first and second piezoelectric transformers with signals whose phases are identical with each other, respectively; a variable oscillation circuit for outputting a variable-frequency voltage to the drive circuits; a cold-cathode tube with one terminal to which the voltage output from the secondary terminal of the first piezoelectric transformer is applied and the other terminal to which the voltage output from the secondary terminal of the second piezoelectric transformer is applied; a current detecting circuit for detecting a current flowing in the cold-cathode tube; a voltage detecting circuit for detecting the voltage applied to the cold-cathode tube; a phase difference detecting circuit for detecting a phase difference between a current signal output from the current detecting circuit and a voltage signal output from the voltage detecting circuit; an active current detecting circuit for detecting an active current flowing in the cold-cathode tube based on the current signal output from the current detecting circuit and the phase difference detected in the phase difference detecting circuit; and an oscillation control circuit for comparing the active current detected in the active current detecting circuit with a predetermined set value and controlling an oscillation frequency of the variable oscillation circuit so that the active current has a value equal to the predetermined set value.
In order to achieve the above-mentioned object, a fourth drive for piezoelectric transformers according to the present invention includes: a pair of current amplifying circuits for amplifying currents converted from AC voltages input thereto having different phases from each other by 180xc2x0, respectively; a pair of step-up transformers for amplifying voltages converted from signals output from the pair of current amplifying circuits and outputting voltage signals whose phases are different from each other by 180xc2x0, respectively; and a pair of piezoelectric transformers each of which includes a piezoelectric body in which a primary side electrode and a secondary side electrode are formed, steps up the voltage signal input to the primary side electrode from one of the pair of step-up transformers, and outputs a voltage signal stepped up from the secondary side electrode.
According to the configurations described above, only an active current flowing in the cold-cathode tube is detected based on the phase difference between an output current and voltage of a piezoelectric transformer, a reactive current caused by the stray capacitance formed between the cold-cathode tube and a reflector can be removed, and the driving of the piezoelectric transformer is controlled accurately so that a constant tube current is obtained. Thus, there can be provided a method and unit for driving a small high-efficiency piezoelectric transformer allowing the cold-cathode tube to have a stable luminance.